Certain partial oxidation products are produced commercially by the oxidation of an appropriate hydrocarbon in the vapor phase over a suitable catalyst. For example, cyclic anhydrides are produced commercially by the vapor phase catalytic partial oxidation of aromatic hydrocarbons, such as o-xylene or benzene, or straight-chain hydrocarbons, such as n-butane or butene, in the presence of an oxygen-containing gas, over a vanadium-containing catalyst. Similarly, nitriles, alkylene oxides, aldehydes and halogenated hydrocarbons are produced by the partial oxidation of appropriate alkanes and alkenes in the presence of selected catalysts. Air is generally used as the oxygen-containing gas, because of its low cost and ready availability. The reaction can be carried out in any suitable reactor, such as a fixed bed, fluidized bed, moving bed, trickle bed or transport bed, and it produces the partial oxidation product, and generally carbon monoxide (CO), carbon dioxide (CO.sub.2), water, and smaller amounts of other partially oxidized by-products. The reaction equipment train generally consists of a reactor, in which the partial oxidation product is produced, a scrubber, in which the partial oxidation product is scrubbed from the reactor effluent gases by means of water or other solvent for the partial oxidation product, and means for further treating the scrubbed effluent gases.
In the past it was common to practice the above-described process on a single pass basis with the conversion of hydrocarbon to the desired product being maximized. This resulted in a low overall efficiency, since the selectivity to partial oxidation product was below the maximum. Consequently, the scrubber effluent gas contained, in addition to unreacted hydrocarbon, considerable amounts of CO and CO.sub.2. These products were usually incinerated, so that the only return realized from them was heat value. In later processes a portion of the scrubber effluent gas was recycled, the conversion of the hydrocarbon feedstock was lowered and the selectivity of hydrocarbon conversion to the desired partial oxidation product was maximized. The remainder of the effluent was purged from the system to prevent the build-up of CO, CO.sub.2 and nitrogen (introduced into the system when air is used as the source of oxygen). These improvements resulted in a reduced "per pass" conversion but the overall efficiency of the process was increased.
Federal Republic of Germany (FRG) Patent Application Disclosure 25 44 972 discloses a maleic anhydride manufacturing process in which the reactor feed comprises C.sub.4 hydrocarbons, air, CO and CO.sub.2. In the process of this patent, maleic anhydride is recovered from the reactor effluent gas stream and a portion of the remaining stream is recycled. This patent also teaches recovering butane by temperature swing adsorption from the non-recycled gas stream and recycling the recovered butane to the reactor.
A major problem associated with the gas phase production of partial oxidation products by the oxidation of hydrocarbons with an oxygen-containing gas is that since the reaction is carried out at elevated temperatures, there is an ever-present danger of a fire or an explosion in the reactor, or the equipment or pipelines associated with the reactor, as a result of the decomposition of unreacted hydrocarbons. The propensity of the hydrocarbons to decompose is enhanced by the presence of catalyst, and the tendency toward decomposition is particularly enhanced in fluidized bed or transport bed reactors. Accordingly, the concentrations of the reactants in the system are maintained such that the mixture is kept outside of the flammability range. Although nitrogen serves to reduce the flammable mixture range when air is used as the source of oxygen for the reaction, the flammable mixture range for hydrocarbon-air mixtures is still quite broad. Consequently, it has been customary to operate gas phase partial oxidation product reactors at low hydrocarbon levels, so that the reaction mixture will remain outside of the flammable range.
U.S. Pat. No. 3,904,652 teaches a gas phase maleic anhydride manufacturing process in which oxygen is used as the oxidizing gas and an inert gas, such as nitrogen, argon, helium or a lower hydrocarbon is fed into a fixed bed reactor with the n-butane and oxygen, the inert gas serving as a diluent to reduce the concentrations of oxygen and butane in the reactor to below the point at which they form a flammable mixture. In the disclosed process, a portion of the gaseous effluent, which contains, in addition to butane, carbon monoxide, carbon dioxide and the inert gas, is recycled. One of the disadvantages of the process of this patent is that recycling carbon monoxide with the other gases increases the fire and explosion hazard at the reactor inlet because carbon monoxide itself is highly flammable.
U.S. Pat. No. 4,352,755 discloses a recycle process for the vapor phase manufacture of maleic anhydride by reacting a straight-chain C.sub.4 hydrocarbon with oxygen in the presence of CO.sub.2. In the process disclosed in this patent the gaseous mixture may contain up to 30 volume percent of carbon dioxide as the inert diluent and contains at least 25 volume percent C.sub.4 hydrocarbon This patent states that at most 2% v/v and more preferably at most 1% v/v of carbon monoxide is present in the oxidation stage. In the process of this patent, the presence of large amounts of C.sub.4 hydrocarbon can render the gas mixture in the system flammable, especially in the region of the reactor outlet.
U.S. Pat. No. 3,868,400, issued to Norton, discloses that in the vapor phase ammoxidation of alkyl-substituted organic compounds the yield of nitrile product can be increased and ammonia and hydrocarbon burn mitigated by incorporating carbon monoxide into the reactant stream to the ammoxidation reaction system.
As is well known, under a given set of conditions of temperature and pressure the flammability of a gaseous hydrocarbon-oxygen mixture is dependent upon the ratio of the gaseous components in the mixture. At very low hydrocarbon concentrations the gas mixture is nonflammable, but at a certain hydrocarbon concentration threshhold level, usually referred to as the lower explosive limit (LEL), the mixture becomes flammable and remains flammable with increasing hydrocarbon concentrations until the hydrocarbon level reaches a certain level, often referred to as the upper explosive limit (UEL) of the gas mixture. The explosive range of a gaseous fuel-oxygen mixture rapidly expands as the temperature of the system increases. Even though it might otherwise be desirable to operate a gaseous partial oxidation product manufacturing process at hydrocarbon concentrations in the explosive range, it is dangerous to do so because of the hazard of fire or explosion in the reactor or associated equipment. The present invention permits optimization of the selectivity and yield of the process, even while operating the process at hydrocarbon concentrations normally falling within the flammable mixture range. In the past, this was not considered possible.